Like one of Harlow's baby monkeys, I keep coming back to mother! I know rental karts may not be a priority, but I hope my input can be put to good use. I have experience in rental karts, so I can offer a qualified opinion here. Since they only have rear brakes, they will spin around 180 degrees almost instantly when there's a hard lock-up, even when the kart is pointed straight. They do become unstable in the game, but not to the degree that they should. AMS 1 modeled it perfectly. They should also have less lateral grip. Basically, they should have a tendency to have the same kind of slow, lazy slides that a race car does on cold slicks.
Hello there, I don't think they will insta spin on lockup, and speaking from experience of various rentals on various surfaces. From my personal experience, they're very easy to drive* and throw around, in fact this "feature" of only rear brakes can be exploited for some fun times, as it gives you simple and easy "throw back end out" whenever you wish for. As you can see, personally I've experienced (both induced and accidental) lockups, yet very small percentage resulted in spinout. *easy to drive and control, ignoring the fact of very heavy steering (depending greatly on driver weight ofc).
The ones I used were old—just months before their retirement. I guess the newer ones may not be as unstable, but the ones I used were. We were even warned about their tendency to spin from lockups beforehand. It was always in cool conditions, so that may have had an effect on the lazy spins that didn't come from lockups.